Electron discharge device



June 23, 1959 w. E. KIRKPATRICK 2,892,122

ELECTRON DISCHARGE DEVICE 2 Sheets-Sheet 1 Filed June 18, 195e Nv QON*/N VEN TOR By W. E. KIR/(PA TR/CK MCM@ AT TORNEV June 23, 1959 w. E.KlRKPA'rRlcK 2,892,122

ELECTRON DISCHARGE DEVICE Filed June 18, 195e 2 sheets-smeet 2 W. E.K/R/(PA TIP/Cl( BV gym C MW A 7' TORNE V -ELEcrnoN nrscnn'non DEvlc'nf YWilliam E. Kirkpatrick, Chatham, NJ., assignor to Bell TelephoneLaboratories, Incorporated, New York, N.Y.,

a corporation ofrNew York n l u p i Application .time 1s, 1956, serialNo; 591,963*

s claims. (ci. 315-12) This invention relates to electrondischarge'device's, and more particularly to such devices ,and circuitsfor counting electrical pulses. n

Multielement vacuum type ,counting tubes to enable counting or steppingat rates' are known. AHowever, the pulse lengths lrequired to insure'correct opera'- tion of such -tubes are often critical. vItthereforewould be desirable to provide a simple tube structure and cir-l cuitwhich would obviate the idisadvantage heretofore encounteredin-multielement counting devices.

It is accordingly an object of the present invention to provide improvedmeans for rapidly counting electrical pulses.

Another object of the invention is to improve the performancecharacteristics of multielement electron dis'- charge devices such ashigh speed electronic counting tubes. y

A further object of the invention is to provide a multielement dischargedevice and an associated circuit capable of yielding one output pulse inresponse to the accumulation of a predetermined-number of input pulses.

A still further object of the invention is toV insure'reliable highspeed counting of electrical signals vof vnoncritical pulse lengths. Y

These and other objects of the invention `are achieved by positioning inan evacuated envelope` a cathode, a control grid, a plurality ofsecondarily emissive target electrodes arranged around the cathode, apluralityof grids associated with each target electrode, and collectorelectrodes compartmentalizing the target electrodes and their associatedgrids. Associated circuitry, cooperates with the novel tube to cause thetarget electrodes to conduct one at a time in sequence in response toelectrical pulses. .g

A counting tube in accordance with theV invention may have any number ofcompartments butfor clarity of presentation a decimal or ten compartmenttube will be assumed. ln such a tube a register or counter in a targetelectrode circuit may advantageously be actuated or energized once inresponse to each ten input pulses. Negative input pulses of anyxlengthwhichv it isV desired to count are applied to the cathode, the controlgrid, to one of the grids associated with each target electrode, and toa flip-nop or switching device. If the input pulses are of positivepolarity, such pulses are applied to the target electrodes and to theflip-flop. An output of the hip-flop is applied to a grid in each of theodd-numbered compartments and the other ip-iiop output,fof oppositepolarity, is applied to a grid in each of the even-numberedcompartments. Each input pulse causes the tube to step or count oneposition. Thus, if counting is commenced with compartment 1, every tenthinput pulse will cause the register in the circuit of compartment toregister or count. Since compartmentlfis initially pulsed and energizedby a starting circuit, it is to ibe noted that the first nine inputpulses may cause the register to count one step. However, in subsequent2,892,122 Patented June 23, 1959 counting cycles ten input pulses arerequired to actuate the register.

The target `electrodes are made of secondarily emissivevmaterialwith asecondary emission ratio greater unity in the tube operating range andconsequently,

.. as ,explained in fuller detail hereinafter, each target has `twostable equilibrium points of operation. One stable point corresponds tothe 01T condition and occurs when the potential difference between thecathode and a target electrode is zero. The second or higher stablepoint corresponds to the on condition and occurs when a target electrodeis positive with respect to the cathode. The target of compartment 1,for example, is initially pulsed to or Aabout the higher stable pointbythe starting circuit. yThis target stabilizes at the higher point andinfso doing primes compartment 2, through an impedance network, forconduction in response to an input pulse. Then, the application of aninput pulse to the tubedoes ineiect three things: Compartment 1 isturned ot, compartment 2 is turned on, and compartment 3 is readied orprimed for conduction in response to the next input pulse.

Thus, a feature of this invention resides in an improved unitizedcounting tube structure and novel circuit associated therewith which usesecondary emission effects of bombarded metal targets to count pulses ofnoncritical lengths at megacycle rates.

Itwis a Afurther feature of this invention that a high speed electrondischarge counting device is provided having-acathode, a control gridsurrounding the cathode,

a plurality ofrradially positioned collector electrodes, and

a secondarilyemissive target electrode between each two collectorelectrodes. Y

A further feature of this invention resides in an elec` trongdischargedevice having a cathode, an array of secondarily emissive targets, anelectrode between adjacent targets and defining distinct compartmentsfor Vthe targets, and a lrst and a second control electrode in each ofthe compartments. Further in accordance with a feature of this inventionaV first group of alternate rst control electrodes connected togetherand a second group of alternate rrst control electrodes are connectedtogether, and an impedance network connects each second controlelectrode to its associated target andv to the target of the priorcompartment in the array. 1 -A 'still further feature of this inventionresides in an electron discharge vdevicerhaving a cathode, a controlelectrode, an array of secondarily emissive targets oppositethe cathode,a iirst and a second electrode between each target, and the controlelectrode, and circuitry for applying `an input pulse tothe cathode,control electrode, and the first and second electrodes, whereby an inputpulse extinguishes the discharge to a rst conducting target, initiatesadischarge to a previously primed second target, and primes a thirdtarget for conduction in response v Y to a next input pulse.

Fig. 2 is a plot of the current-voltage characteristic u of 4each of thetarget electrodes employed in the electr'on discharge device and circuitof Fig. 1;

, Fig.- 3 is a plot of the output voltage of the starting pulse sourceas a function of time; and

eilig. y4is a plot of the current-voltage characteristic.

of each of the target electrodes employed in an electron dischargedevice and circuit similar to that shown in Fig. 1 but wherein the inputpulses are applied to the target electrodes.

Referring now to the drawing, Fig. 1 illustrates an elect-ron dischargedevice or counting tube 20 comprising an evacuated envelope or enclosure21, for example, of glass or other suitable material, having therein acentrally positioned cylindrical cathode 22 which maybe of theindirectly heated type, the heater not being shown in the drawing. Thecathode 22 carries on its outer cylindrical surface a coating ofemissive material which may be of any conventional type. Surrounding thecathode 22 and spaced equidistantly therefrom at all points is acylindrical control grid 23.

Annularly positioned around the cathode 22 and the control grid 23 areradially extending collector electrodes 24, a plurality of grids 25,hereinafter referred to as A grids, a second plurality of grids 26,hereinafter designated as B grids, and target electrodes 27. interposedbetween each target electrode 27 and its associated A grid 25 or B grid26 is a grid 28 designated as a G grid.

The collector electrodes 24 serve to compartmentalize the counting tube20 and, thus, the tube may be considered as comprising within `a singleenvelope 21 a plurality of discharge devices which can be interconnectedwith external circuitry for high speed counting. For counting in thedecimal system, for example, the counting tube 20 may comprise tencompartments, each including a target electrode 27 and associated grids.The various electrodes are held in proper spaced relationto one 'anotherby supports, not shown, made of electrically nonconducting material,such as mica.

Fig. 1 illustrates a ten compartment tube wherein lthe target, G gridand A grid elements of compartment 1 are respectively designated 27-1,28-1, and 25-1. The-target, G grid, and B grid elements of compartment 2are respectively designated 27-2, 28-2, and 26-2. lThe elements in theother compartments are similarly numbered, with the dash numberassociated with each element indicating its compartment location.

In actual embodiments of counting tubes according to this invention anindividual lead extends through the envelope 21 from each targetelectrode 27, from each G grid 28, from the cathode 22, and from thecontrol grid 23 to appropriate external terminals, not shown. A singlelead extends through the envelope to an appropriate external terminal,also not shown, from the A grids 25 which are all electricallyinterconnected within the tube and, similarly, one lead extends to asuitable external terminal, not shown, from the B grids 26 -which arealso all electrically interconnected within the tube.v Further, all thecollector electrodes 24 are internally connected together and a lead 50connects them tov an external source of positive potential Vc. In otherspecic embodiments according to the invention some-ofl the circuitelements may be placed within the counting tube and the number ofindividual leads extending through the envelope thereby reduced.

Fig. 1 also shows the counting tube 20 connected into a suitablecounting circuit. Each G grid 28 is connected through a resistor 30 anda capacitor 31 in parallel via lead 56 to the negative terminal of aD.C. source such as the battery 32, hereinafter referred to as EG. Acapacitor 18 is connected in parallel with EG. The positive terminal ofEG is connected through an input resistor 33 to ground. The cathode 22is connected to the positive side of EG by leads 53 and 54, and is alsoconnected to the positive terminal of a D.C. source such as the battery34. The negative terminal of the battery 34 is connected to the controlgrid 23 by lead 55, and a capacitor 35 is connected in parallel with thebattery 34.

The target electrodes 27 are each connected through a resistor 29,hereinafter referred to as the target resistor, to ground, and eachtarget .electrode is connected to its associated G grid through aresistor 36. Further, a resistor 37 connects the ungrounded side of eachtarget resistor 29 to the G grid 28 of the adjacent clockwisecompartment.

The external terminal, not shown, for the A grids is connected to oneoutput terminal 38 of a ip-op or switching device 80, and the otheroutput terminal 39 of the flip-Hop 80 is connected to the externalterminal, not shown, of the B grids. 'At any one time flip-nop 80supplies a bias voltage of one polarity, with respecttoA the cathode 22,to the A grids and a bias voltage of the opposite polarity to the Bgrids.

The input terminal 40 of the counting circuit of Fig. 1 is directlyconnected by lead 57 to a source 90 of input pulses by lead 53 to thecounting tube cathode 22, and by lead 52 to the input of the Hip-flop80. Capacitors 41, 42 are connected respectively between the ilip-opinput lead 52 Vand its output terminals 38, 39.

A lstarting pulse source 75 is connected to one terminal of a normallyopen switch 43 and the other terminal of the switch 43 is connectedthrough a capacitor 44 to the ungrounded side of the target resistor 29of, for example,

compartment 1.

i A register or counting device 70 having approximately the resistanceof a target resistor 29 may be inserted in place of such a resistor inany target electrode 27 circuit. Such a counting device V7|) is shown inFig. l

1 in the target electrode circuit of compartment 10.

potential is above Vo-i-AV.

11n Fig. 2, the net target electron current is shown plotted as afunction of the target potential for the case where the targets 27 aremade of a material in which the secondary emission ratio exceeds unitywithin the counting tube' 20 operating range. For the solid line curve92, Vk is the potential of the electron source or cathode 22, Vo yis thevalue of bombarding or target potential at which the secondary emissionratio of the target material iseqiial to unity, and Vc is the potentialof the collector electrodes24.` Vo of Fig. 2 may-be about 50 volts in aspecific embodiment of this invention. A load line 45 represents atarget resistor 29, and intersects the plotted curve at two stablevalues of target potential. rIhese values are Vk, the cathode potential,and the potential V1.' If the cathode potential is dropped by somevoltage V to Vk' then the target current-Voltage characteristic willchange to the dashed curve 93 of Fig. 2 `and the stable ypotentialvalues will then be Vk and V1', as indicated by the load line 46.

If 'a target potential is not at one of the two stable values and if thetarget is being bombarded by electrons from a cathode at potential Vk,the target will charge to potential Vk if the target potential is belowVO-l-AV or the target will charge to potential V1 if the target Thissame type of action applies `to the-dashed curve with respect to thepotential V0l-AV when the `cathode is at potential Vk'.

This action of charging to one of the two stable potential -values may-be seen from the following consideration.A At'low primary energies ortarget potentials, the

' secondary emission ratio is less than unity, indicating that thenettarget electron current is positive, and the targetbeing bombarded isassuming a negative charge fromfthe aquisition of electrons. Thebombarded target willaccumulate negative Vcharges until it finallyreaches the cathode potential, a stable. point.

IWhen .the ytarget potential is increased to a critical voltage V0, thesecondary emission ratio of the target materialbecornes equal to unity,and the net current to the target lwould be zero if lthe impedance inthe target circuit were infinite. The presence of a nite load impedanceint the target vcircuitprovides an additional path for velectron ,flowso that the potential at which the effecti-vetargetsecondary emissionratio becomes equal to unity is increased by AV. AV is the potentialdifference besects the curve of Fig. 2 immediately above V0. At pogesamtfentialsslightly higher than VTI-AV, the'etectivel target secondaryemission ratio exceeds unity and-the net electron current -to thevtarget'isnegativ'e The target accumulates positive charges until itreaches an equilibrium potentialY determined by the intersection ofthetarget resistor load line and the target characteristic. Thus, thecathodev potential and theiintersection potential'are the two stablepoints of operation fora bombarded secondarily emissive target element.

With la particular target, for example 27-1, at potential V1 and allother targets at potential Vk, an electrical pulse, whose length,amplitude andshape characteristics are discussed below, applied at theinput terminal 40 will cause the target electrodeof the compartment onepositionl clockwise of thev one at V1, e.g. that of compartment 2, tocharge to V1 while -all other targets stabilize at potential Vk. Thus,the application of a pulse to the input resistor 33 will cause the tube20 to count, or step, one position clockwise.

The operation `of this specic illustrative embodiment of a countingtubeZl) and circuit in accordance with my invention is as follows:

" The tube, even when its heater is energized, is initially in the olfcondition, i.e., allof the target electrodes are atpotential Vk, thelower of the two stable potential values referred to above. The readyingor starting circuitcomprises the starting pulse source 75, the switch43, and the capacitor 44. When the switch 43 is closed, the startingpulse source 75 applies a positive pulse, Vlike that shown in Fig. 3,through the capacitorf44 to the target resistor 29 of compartment 1. Theamplitude a of thisl pulse must be greater than VO-i-AV. The iiip-op 80is so arranged that in the absence of an input pulse the A grids V25 arepositive and the B grids 26 with respect to the cathode 22. Electronscannot pass the negative B grids 26 and, thus, only the odd numberedtarget compartments are capable of being triggered. The resistors 29,30, 36 and 37 are so chosen that the starting pulse voltage appearing atthe target electrodes of compartments 3 and 9 issubstantially less thanVO-i-AV, and the target electrodes of odd-numbered compartments moreremotely positioned from compartment 1 than compartments 3 4and 9 are atstill lower potentials. Thus, only the target of compartment 1 chargesto potential V1'while `the remaining targets 27 remain at or nearpotential V11.l Also, the resistance values 29, 30, 36 and 37 are sochosen that only the G grids 28 of compartments 1 and 2 aresubstantially positive withV respect to the cathode 22 when the targetof compartment 1 is at potential V1. The slight positive bias on theother G grids is removed by battery 32. Although the G grid 28-2ofcompartment 2 is positive, no electrons reach the target electrode27-2 of that compartment because, as noted above, its B grid 26-2 isvnegative with respect to the cathode 22.

The counting device having been started or readied, assume'nowthat anegative pulse, for example, is applied ttheinput terminal 40. The pulseamplitude V' must be greater than V'+AV'-Vk', and further, it isnecessary that the applied pulse rise fast enough for the targetpotential ltorise above V-|AV againstjthe negative charging `actionbelowVDH-AVT' Also, Vthe pulse length should at least be such that before theremoval of the pulse the target electrode has charged to a point on thedashed curve of Fig. 2 corresponding to a target potential greater thanVD-I-AV. A 'IhenQat the pulse removal, when the' operation changes backto the solid curveof Fig. 2, the target, being at a potential greaterthan V0+AV, will stabilize at V1. Otherwise, neither pulse length norpulse shape has anyadverse efect on reliable high speed counting action.i

The cathode 22, the control grid 2,3, and the G grids 28y are all pulsednegative by a negative input pulse. This causes theloperatingcharacteristic shown in Fig. 2 tochan'ge from the solid to the dashedcurve. Just' 'are negative before the application of the above assumedinput puls, the target of compartment 2 wasat potential Vk. Vk targetpotential with respect tothe dashed `curve is, as Fig. 2 clearly shows,a value of target voltage greater than V+AV1 and so the target ofcompartment 2 will charge to the higher stable target voltage V1 uponthe application of the input pulse. Electronsreach only the targetelectrode 27-2 of compartment 2 upon the application of the pulsebecause, as explained above, conduction in the target electrode circuitof compartment 1 maintained only the G grids of compartments 1 and 2 atpositive potentials with respect to the cathode 22. Further, the inputpulse to the ip-op reverses the potentials on the A 25-1 and B 26-2grids so that conduction in the target electrode circuit of compartment1 is cut off at approximately the same time that compartment 2 isrendered able to conduct.

When the negative input counting pulse, i.e. the'voltage V', is removed,the operating curve once morebecomes the solid curve of Fig. 2 and thetarget 'electrode 27-2 of compartment 2 then charges from potential V1'to V1. Conditions at compartment 2 are then as they were at compartment1 previous to the application of the input pulse, and the targetelectrode 27-2 of cornpartment 2 remains at potential V1 While all othertarget electrodes are at potential Vk.

In the specific embodiment of the invention shown in Fig. l, every tenthinput pulse with, as noted above, the exception of the first countingcycle, causes the target electrode circuit of compartment 10 to conductand its counting device 70 to register. This invention is, of course,not intended to be limited to a counting tube having therein tencompartments or sections. Furthermore, despite the references toclockwise counting, the counting tube and circuit described herein canbe easily arranged so that counting is counter clockwise.

The capacitors 31 permit negative input pulses to pass on to the G grids28. Thus, the potential diierence between the cathode 22 and the G grids28 is not changed by application of negative input pulses to the cathode22. Further, the capacitors 31 serve to hold the G grid D.C. levels atprevious values for a time. This is Vdesirable since, for example, the Ggrid 28-2 of compartment 2 is kept positive before the application of aninput pulse by the action of electrons bombarding the targe electrode 27of compartment 1. Whenthe input pulse is applied, the current incompartment 1 may shut off rapid ly and the voltage at the G grid ofcompartment Zmay drop too low to permit electrons to pass to the targetelectrode 27 of that compartment. The presence of the capacitors 31holds the G grid D.C. levels for the time constant of the resistor 30,capacitor 31 circuits, and thereby gives the .counting tube 20 time tocount, or step.

The D.C. source 34, which may be an adjustable or variable supply,adjusts the level of current iow in the counting tube, and may also beused to cut off the tube. The capacitor 35, in parallel with the D.C.source 34, insures a low A.C. impedance path from the input terminal 40to the control grid 23. Similarly the capacitor 1S by-passes the D.C.source 32. ,TheY capacitors 4l, 42 provide negative input pulses with alow A.C. impedance path to the A 25 and B 26 grids. Thus, the A 25 and B26 grids are pulsed negative with the cathode 22, control grid 23, and Ggrids 28. It is to be noted that only the target and thecollector'electrodes 'are not so pulsed by negative inputs.VV Y T Sincethe output appears in a target electrode circuit, separation of inputand output pulses is obtainedwhen counting negative pulses with thenovel tube and .circuit of this invention. It is possible, however, tocause the tube to count by application of a positive input pulse to allthe target electrodes andto the ip-op 80. For counting positive pulses,the circuit of Fig. 1 must be modified so that a-lead extends from thejunction ,of the input terminal 40 and the input resistor 33 to eachtarget electrode 27 through a capacitor, not shown, and so that the-lead91 is grounded and not joined to the aforementioned junction. Further,the flip-flop Lcapacitors 41, 42 and the capacitor 35 are omitted.However, the input terminal 40 and the input resistor 33 remainconnected `to the input of the flip-hop.

The operationof this device when vtriggered by positive pulses to theflip-flop and the target electrodes is illustrated by Fig. 4. Since thepotentials Vk, Vc, and Vo, which represent respectively the cathodepotential, the collector potential, and the potential at which secondaryemission of the target electrode material is unity, remain unchanged,the operation is along the single curve 94. The load line 60 intersectsthe operating characteristie at the two stable values of targetpotential Vk and V2. I n order to efect reliable high speed countingaction, the following conditions must be satisfied. First, the vrisetime of the applied pulse V must be suiciently short so that the targetpotential mayrise above Vo-l-AV against the negative charging actionbelow Vo-l-AV. Second, the pulse length should at least be such thatbefore the removal of the pulse, the target electrode has charged to apoint onthe curve of Fig. 4 so that at the removal of the pulse, thetarget will not be carried back along the curve past Vo-i-AV. Third,while the amplitude of each positive input pulse V" must be greater thanVo-l-AV, the pulse amplitude should not be so great as to carry thetarget electrode potential back past Vo-i-AV upon removal of the pulse.Fourth, the potential difterence between V2 and Vo-l-AV must be greaterthan VD-l-AV minus Vk in order to prevent the removal of the pulse Vfrom carrying the potential of the target electrode back past Vo-l-AVand back to stability at Vk. Conditions 3 and 4 may be easily met byadjusting the common collector potential Vc accordingly.

The application of such a positive input pulse of amplitude V" willcause an energized compartment, for example 1, to cut off due to theflip-dop 80 switching the A and B grid voltages. Electrons cannot passthe negative A grids 25, and thus only the even numbered compartmentsare capable of being triggered. In all the even numbered compartmentsthe B grids 26 will be positive and the target electrodes will have beenpulsed to a potential greater than Vo-l-AV. However, only in compartment2, adjacent in a clockwise direction to the previously energizedcompartment 1, will the G grid voltage be of such a positive Value so asto permit conduction. All the other G grids except that of compartment 1will be at zero or a slightly negative bias due to voltage dividing bythe resistors 29, 30, 36 and 37, voltage holding by the capacitors 31,and the action of the battery 32. Therefore, conduction will take placeonly in compartment 2, and its target Velectrode will charge toward thestable potential V2. If the pulse V is of a nature as set forth above,its removal will not affect the charging action of the target electrode,and the target will stabilize at V2. Thus, the application of a positivepulse to the target electrodes Z7 and to the flip-flop 80 will cause thetube 20 to count, or step, one position clockwise.

Although specific embodiments of this invention have been shown anddescribed, it will be understood that they are but illustrative and thatvarious modifications may be made therein without departing from thescope and spirit of this invention.

What is claimed is:

1. An electron discharge device comprising a cathode, an array Aofsecondary emitting targets opposite said cathode, a first and secondelectrode between each target and said cathode, means for applying ablocking potential to alternate of said first electrodes to preventpassage of electrons therethrough from said cathode to the assoc iatedtargets, means normally applying a first potential bias to each of saidsecond electrodes, and impedance means for applying a second potentialbias to a second electrodeon occurrence of a discharge at the pn'ortarget; in s aid array. L

2. A high speed electroniccounting tube comprising an evacuated envelopecontaining means for producing an electron beam, a control grid, aplurality of secondarily, emissive target electrodes, a plurality ofgrids associated with each target electrode, and collector electrodescompartrnentalizing each target electrode and its associated grids.

3. An electron discharge device comprising a cathode, an array ofsecondary electron emitting targets, electrode means between adjacenttargets and defining distinct cornpartments for said targets, a firstand second control electrode in each of said compartments between saidcathode and the target in the compartment, means connecting a rst groupof alternate first electrodes together and a second group of alternatefirst electrodes together, and impedance means connecting each secondelectrode .to its associated target and to the target of the priorcompartment in the array.

4. A high speed electronic counting circuit comprising an evacuatedenvelope, means for producing an electron beam, a control grid, aplurality of annularly arranged secondarily emissive target electrodes,a plurality of grids associated with -each target electrode, collectorelectrodes compartmentalizing each target electrode and its associatedgrids, an input terminal, D.C. potential means, an electronic switchingdevice, a parallelresistorcapacitor network connected between thenegative terminal of said potential means and the grid closest to eachtarget electrode, the positive terminal of the potential means connectedto said input terminal and to the beam producing means, and through aparallel capacitor-biasing means network to the control grid, said inputterminal connected to a plurality of grids within the envelope throughthe electronic switching device, and a resistor network interconnectingeach target electrode and its closest grid with the target electrodes ofadjacent compartments.

5. An electron discharge device comprising a cathode, a plurality ofsecondarily emissive target electrodes positioned in an array, aplurality of control grids associated with each target electrode,electrode means compartmentalizing each target electrode and itsassociated grids, means including a resistor-capacitor network connectedto each grid closest its target electrode for maintaining a. rstpotential bias on said grid, and means connecting said grid to the nextprior target in the array for maintaining a second potential bias onsaid grid on occurrence of a discharge at said prior target.

6. An electron discharge device comprising a cathode, a plurality ofsecondary emissive target electrodes positioned opposite said cathode, aplurality of control electrodes individual to each target electrode andinterposed between said target electrodes and said cathode, first meansconnecting together a first group of control electrodes of alternatetargets, second means connecting together a second group of controlelectrodes of the remaining alternate targets, and a bi-stable circuithaving -one output connected to said first means and the other outputconnected to said second means for alternately applying a bias potentialto said controlv electrodes to inhibit passage of electrons from saidcathode to the associated targets.

7. An electron discharge device in accordance with claim 6 furthercomprising a source of input pulses, means connecting said source tosaid cathode and to said bi-stable circuit, and means shunting saidbi-stable circuit for applying pulses from said source directly to saidcontrol electrodes.

8. An electron discharge device comprising a cathode, an array ofsecondaryemitting targets opposite said cathode, a first and a secondcontrol grid individual to each of said targets and interposed between,said cathode yand each of said. targets, .a source 0f 4Signal pulses,means I0?.

2,892,122 9 10 applying pulses from said source simultaneously to saidReferences Cited in the file of this patent cathode and said rst andsecond control grids, means UNITED STATES PATENTS for normally applyinga rst bias potential to all of and impedance means for applying a secondbias potential to a second control grid on occurrence of a discharge atthe next prior target electrode in the array.

